Process for measuring bond-line thickness

ABSTRACT

A process for measuring the thickness of an insulating material. The process includes providing a device used to measure capacitance, and electrically connecting the capacitance measuring device to a heat sink and an electrical, heat-generating component. The thickness of the insulating material is determined by measuring the capacitance of the insulating material according to the formula; 
         B=ε   r ε 0   A/C,    
     where B is the thickness of the insulating material, C is the capacitance, A is the area of the heat generating component, ε 0  is the permittivity of free space and ε r  is the relative dielectric constant of the insulating material.

FIELD OF THE INVENTION

The invention is related to a process for measuring the thickness of aninsulating material. The invention is also related to an apparatus andpackage structure that provides for capacitance measurements so that thethickness of the insulating material can be determined.

BACKGROUND OF THE INVENTION

Heat dissipation from active semiconductor devices has become a majortechnical challenge in achieving continued increases in performance andfunction.

The thermal resistance between the backside of a flip chip and the heatspreader attached to it decreases as the bulk thermal conductivity ofthe thermal interface material (TIM) increases and as the bond linethickness decreases. Therefore, accurate measurement of the bond lineover the entire chip area is essential to design and predict the thermalresistance. Conventional methods to measure bond line includecross-sectioning; laser profiling before and after lid attach; and lidremoval after cure combined with z-scope measurement through theadhesive thickness.

The most common method is cross-sectioning which is both destructive andtime consuming providing only a few points of measurement over the chiparea. First the module is encapsulated to fill any open cavities in thepackage. After cure is complete, a diamond saw is used to cut the sampleand expose a cross section that could be along an edge or diagonal. Thissample piece is then encapsulated again to facilitate the hand grindingand polishing preparation that is required to reveal clean interfacesthat allow measurement of the bond line. However, because of the largedifference in the hardness of materials that include silicon, copper andpolymers, materials are torn out or smeared during grinding andpolishing therefore making it difficult to discern the exact interface.After the polishing sequences, the bond line dimension is measured atseveral points along the section line using an x-y microscope or digitalimage analysis. It takes several days to complete cross-sectionalanalysis on a few modules, so this method is viable for development workand failure analysis but not for real-time process characterization orline monitoring.

Z-scope measurements provide quicker turn around compared tocross-sectioning. Measurements are required in several spots tounderstand bond line variation over the chip area. One method is to dryplace a heat spreader onto a chip and laser scan the lid surface. Athermal interface material is then applied to either the chip or heatspreader and the heat spreader is mated to the chip with a force and adwell time. After the force is released, a second laser scan is made.The z-direction difference between the two scans is a measure of thebond line. The bond line is a difference of between 2000 and 2050microns for only 50 microns. Typically this method is used to measurewet or uncured bond lines to make sure the process is in control. Forprocesses that require pressure during cure, this method could only beused after cure as laser scanning of the heat spreader is not possiblein a force loaded cure fixture.

A second z-scope method that is destructive entails removing the heatspreader after cure. If the thermal interface material debonds very nearone of the interfaces, and near full thickness remains at the othersurface, material can be scratched away in a small area and a z-focusmicroscope can be used to measure the bond line in several areas byfocusing on the surface of the thermal interface material and then onthe substrate to which it is attached. This method is reasonable andquick for bond lines that are relatively thick (>75 microns).

Flip chips attached to organic carriers are known to warp because of theCTE mismatch between silicon and laminate (2.8 vs 12-18 ppm/C). Thus thebond line between the chip and heat spreader is not uniform. Dependingon the package, the chip bending ranges between 50 to 100 microns.Measuring the bond line over the entire chip area is not practical viacross sectioning.

In order to meet thermal resistance design targets, bond lines aredecreasing to 25 microns and lower. Thus, it is becoming even moredifficult to make accurate measurement of the bond line.

SUMMARY OF THE INVENTION

The invention is directed to a process for measuring the thickness of aninsulating material. The process includes providing a device used tomeasure capacitance, and electrically connecting the capacitancemeasuring device to a heat sink and an electrical, heat-generatingcomponent. The insulating material is disposed between the heat sink andthe heat-generating component. The thickness of the insulating materialis determined by measuring the capacitance of the insulating materialaccording to the formula;

B=ε _(r)ε₀ A/C,

where B is the thickness of the insulating material, C is thecapacitance, A is the area of the heat generating component, ε₀ is thepermittivity of free space and ε_(r) is the relative dielectric constantof the insulating material.

The process can be used to measure the bond-line thickness of adielectric material of a semiconductor package. In such a case, theprocess includes providing a semiconductor package, which includes adielectric material disposed between a semiconductor chip and a heatspreader, and providing a device used to measure capacitance. Thecapacitance can be measured by providing at least one lead of thecapacitance measuring device in electrical contact with the heatspreader, and at least one other lead of the capacitance measuringdevice in electrical contact with the semiconductor chip.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become apparent uponconsideration of the following detailed description of the inventionwhen read in conjunction with the drawings, in which:

FIG. 1 depicts a cross sectional view of a semiconductor package of theinvention;

FIG. 2 depicts cross sectional view of another semiconductor package ofthe invention;

FIG. 3 depicts a cross sectional view of yet another semiconductorpackage of the invention;

FIG. 4 depicts a perspective surface view of a heat spreader with aplurality of conducting electrodes; and

FIG. 5 is a photograph of a semiconductor package of the invention.

One of ordinary skill in the art understands that the above describeddrawings are exemplary embodiments of the invention, and that theclaimed invention is not limited by such exemplary embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to a process for measuring the thickness of aninsulating material. The process includes providing a device used tomeasure capacitance, and electrically connecting the capacitancemeasuring device to a heat sink and an electrical, heat-generatingcomponent. The insulating material is disposed between the heat sink andthe heat-generating component. The thickness of the insulating materialis determined by measuring the capacitance of the insulating materialaccording to the formula;

B=ε _(r)ε₀ A/C,

where B is the thickness of the insulating material, C is thecapacitance, A is the area of the heat generating component, ε₀ is thepermittivity of free space and ε_(r) is the relative dielectric constantof the insulating material.

In one embodiment, the insulating material is a dielectric material andthe heat-generating component is a semiconductor chip.

The process can be used to measure the bond-line thickness of asemiconductor package. In such a case, the process includes providing asemiconductor package, which includes a dielectric material disposedbetween a semiconductor chip and a heat spreader, and providing a deviceused to measure capacitance. The capacitance can be measured byproviding at least one lead of the capacitance measuring device inelectrical communication (for example, an alternating current signalcontact) with the heat spreader, and at least one other lead of thecapacitance measuring device in electrical communication with thesemiconductor chip. The bond-line thickness (B) of the dielectricmaterial is determined by measuring the capacitance of the semiconductorpackage according to the formula;

B=ε _(r)ε₀ A/C,

where C is the capacitance, A is the area of the semiconductor chip, ε₀is the permittivity of free space and ε_(r) is the relative dielectricconstant of the dielectric material.

In one embodiment of the semiconductor package, the at least one lead inelectrical communication with the semiconductor chip is electricallyconnected to a conductive plate. At times, a chip carrier is disposedbetween the conductive plate and the semiconductor chip of the package.The chip carrier can include a plurality of electrical leads thatelectrically connect the conductive plate to the semiconductor chip.

In another embodiment of the semiconductor package, the heat spreaderincludes one or more conducting electrodes that extend from one surfaceto the opposite surface of the heat spreader. The conducting electrodesare electronically insulated from the heat spreader. The electronicconnections of the package can include at least one electricalconnection from the capacitance measuring device to the one or moreconducting electrodes. The electronic connections of the package canalso include at least one electrical connection from the capacitancemeasuring device to a region of the heat spreader that is electricallyinsulated from the one or more conducting electrodes. At times, theplurality of electrical leads that electrically connect the conductiveplate to the semiconductor chip include carrier pads.

In another embodiment of the semiconductor package, at least oneelectrical connection from the capacitance measuring device to the oneor more conducting electrodes includes a plurality of conductingelectrodes. Preferably, each of the plurality of electrodes is inelectrical communication with a single capacitance measuring device ormultiple capacitance measuring devices.

Some of the commercial benefits of the capacitance measuring processdescribed include the following. The process can be non-destructive.Also, the process provides relatively fast diagnostic information and ishighly accurate.

Accordingly, the process can be operated in real time before, during, orafter the cure process. Also, with a modified heat spreader containingmultiple conducting electrodes that are electrically isolated from theheat spreader, the bond-line can be mapped across most, if not all,areas of the semiconductor chip. In this manner, heat spreader tilt canalso be measured. This method is very useful to help develop andoptimize a heat spreader attachment process as well as provide processcontrol data to support achieving thermal resistance targets.

FIG. 1 shows a cross section view of a semiconductor package 10 with adielectric layer of thermal interface material 12 between the siliconchip 14 and a heat spreader 16. A capacitance meter 18 is connected tothe silicon chip and the heat spreader allowing measurement of thecapacitance across the dielectric layer. In FIG. 1, a direct electricalconnection to the silicon chip 14 is shown, which may not always bepossible or convenient.

The thickness of the dielectric layer, also referred to as the bondline, B, can be calculated using the following formula for parallelplate capacitance, B=ε_(r)ε₀A/C, where C is the capacitance, A is thearea of the silicon chip, ε₀ is the permittivity of free space (8.85e-12Farads/meter), and ε_(r) is the relative dielectric constant of thedielectric layer. The relative dielectric constant is a materialproperty that is often listed in data sheets provided by manufacturers.

Alternatively relative dielectric constant can be measured using aparallel plate apparatus in which the distance between the electrodes isknown. The distance or space between the two electrodes is then filledwith a dielectric material with an unknown relative dielectric constant.The measured capacitance thus allows one to calculate the relativedielectric constant using the parallel plate capacitance formula above.It is desirable that the bond line capacitance measurement be performedat the same frequency as that used by the process as the relativedielectric constant is often frequency dependent.

The actual frequency used in the process is not critical. For example,the measuring capacitance can be achieved using a commercially availablemeter (Agilent 4263B) operating at 10 Khz.

In another embodiment, FIG. 2 depicts package structure 21 in which oneend of a capacitance meter 18 connects to a conductive plate 20 used tosupport the silicon chip. The silicon chip 14 can be shown soldered(depicted as solder balls 24) to a chip carrier 22. The chip carrierincludes internal wiring that connects the chip circuitry to carrierpads 28 on the bottom of the carrier. As shown, a conductive layer 26between the carrier pads 28 and the conductive plate 20 is used toprovide adequate electrical contact to the silicon chip 14. For example,water can be used as the conductive layer. It is to be understood thatone can also use an apparatus in which the carrier pads are in directcontact with the conductive plate.

In another embodiment, FIG. 3 depicts a package structure 30 in which aheat spreader 32 is modified to allow a localized region of thedielectric layer 12 to be measured rather than the entire silicon chiparea. For example, one can drill a hole through a standard spreader atthe desired location and insert a conducting electrode 34 that iselectrically insulated 36 from the main spreader body 32. The surfacecan be machined or hand polished to remove excess electrode andinsulating materials and cured adhesive, if used, to restore the smoothplane surface of the spreader. In this case the value of A in theparallel plate capacitance formula is the area of the conductingelectrode. As shown in FIG. 3 a meter such as the Agilent 4263B that hasfour terminals measurement capability can be used. The outer shields onthe four leads are connected together near the device under test andconnected to the body of the spreader so that capacitance across theinsulator 36 is not sensed. By using such a connection configuration,the stray capacitance contribution can be about 0.01 pF or less. Forcomparison, the capacitance of a typical 1 mm diameter conductingelectrode, with a 25 micron bond line having a relative dielectricconstant of 48 is about 14 pF.

In yet another embodiment, the apparatus of FIG. 4 provides fordetermination of the bond line thickness at various locations across thechip surface by placement of a plurality of conducting electrodes 40into a heat spreader 42. Using multiple meters, sequentially attachingeach electrode to a single meter, or relay switching multipleconnections to a single meter allows determination of the bond linethickness at different locations on the chip.

Important mechanical properties, e.g., the bending stiffness and thermalexpansion coefficient, of the electrode-modified heat spreadersdescribed above are only minimally affected by the insertion of theelectrodes. This is particularly true if the electrodes are made of thesame material as the spreader. This can be important since curing andpost-cure stressing usually involves substantial temperature excursions,and it is desired that the test units respond in the same way asstandard product.

FIG. 5 illustrates an embodiment in which the conducting electrodes 50are patterned onto a thin sheet 52 (such as copper Kapton) which isinserted between the spreader 54 and the TIM. Patterned leads 56 to eachelectrode are brought out laterally beyond the edge of the spreader 54to allow connection to a capacitance meter.

The invention is also directed to a semiconductor package. Thesemiconductor package includes a dielectric material disposed between asemiconductor chip and a heat spreader in combination with a device usedto measure capacitance. Also, at least one lead of the capacitancemeasuring device is in electrical communication with the heat spreaderand at least one other lead of the capacitance measuring device is inelectrical communication with the semiconductor chip.

In one embodiment, the semiconductor package that at least one lead inelectrical communication with the semiconductor chip can be electricallyconnected to a conductive plate. Also, the semiconductor package caninclude a chip carrier disposed between the conductive plate and thesemiconductor chip. The chip carrier includes a plurality of electricalleads that electrically connect the conductive plate to thesemiconductor chip. The plurality of electrical leads that electricallyconnect the conductive plate to the semiconductor chip can includecarrier pads.

In another embodiment, the semiconductor package includes a heatspreader that includes one or more conducting electrodes that extendfrom one surface to the opposite surface of the heat spreader. Theconducting electrodes are electronically insulated from the heatspreader. The semiconductor package includes at least one electricalconnection from the capacitance measuring device to the one or moreconducting electrodes. The semiconductor package can also include atleast one electrical connection from the capacitance measuring device toa region of the heat spreader that is electrically insulated from theone or more conducting electrodes.

1. A process for measuring the thickness of an insulating materialcomprising: providing a device used to measure capacitance, andelectrically connecting the capacitance measuring device to a heat sinkand an electrical, heat-generating component, wherein the insulatingmaterial is disposed between the heat sink and the heat-generatingcomponent; and measuring the capacitance of the insulating material toprovide the thickness B according to the formula, B=ε_(r)ε₀A/C, where Cis the capacitance, A is the area of the heat generating component, ε₀is the permittivity of free space and ε_(r) is the relative dielectricconstant of the insulating material.
 2. The process of claim 1 whereinthe insulating material is a dielectric material and the heat-generatingcomponent is a semiconductor chip.
 3. A process for measuring thebond-line thickness of a semiconductor package, the process comprising:providing the semiconductor package which includes a dielectric materialdisposed between a semiconductor chip and a heat spreader; providing adevice used to measure capacitance, wherein at least one lead of thecapacitance measuring device is in electrical communication with theheat spreader and at least one other lead of the capacitance measuringdevice is in electrical communication with the semiconductor chip;measuring the capacitance of the dielectric material to provide thebond-line thickness B of the dielectric material according to theformula, B=ε_(r)ε₀A/C, where C is the capacitance, A is the area of thesemiconductor chip, ε₀ is the permittivity of free space and ε_(r) isthe relative dielectric constant of the dielectric material.
 4. Theprocess of claim 3 wherein the at least one lead in electricalcommunication with the semiconductor chip is electrically connected to aconductive plate.
 5. The process of claim 4 further comprising providinga chip carrier disposed between the conductive plate and thesemiconductor chip, wherein the chip carrier includes a plurality ofelectrical leads that electrically connect the conductive plate to thesemiconductor chip.
 6. The process of claim 3 wherein the heat spreadercomprises one or more conducting electrodes that extend from one surfaceto the opposite surface of the heat spreader, wherein the conductingelectrodes are electronically insulated from the heat spreader.
 7. Theprocess of claim 6 further comprising providing at least one electricalconnection from the capacitance measuring device to the one or moreconducting electrodes.
 8. The process of claim 7 further comprisingproviding at least one electrical connection from the capacitancemeasuring device to a region of the heat spreader that is electricallyinsulated from the one or more conducting electrodes.
 9. The process ofclaim 5 wherein the plurality of electrical leads that electricallyconnect the conductive plate to the semiconductor chip include carrierpads.
 10. The process of claim 6 wherein the at least one electricalconnection from the capacitance measuring device to the one or moreconducting electrodes includes a plurality of conducting electrodes,wherein each of the plurality of electrodes is in electricalcommunication with the a single capacitance measuring device or multiplecapacitance measuring devices.
 11. A semiconductor package comprising adielectric material disposed between a semiconductor chip and a heatspreader in combination with a device used to measure capacitance,wherein at least one lead of the capacitance measuring device is inelectrical communication with the heat spreader and at least one otherlead of the capacitance measuring device is in electrical communicationwith the semiconductor chip.
 12. The semiconductor package of claim 11wherein the at least one lead in electrical communication with thesemiconductor chip is electrically connected to a conductive plate. 13.The semiconductor package of claim 12 further comprising a chip carrierdisposed between the conductive plate and the semiconductor chip,wherein the chip carrier includes a plurality of electrical leads thatelectrically connect the conductive plate to the semiconductor chip. 14.The semiconductor package of claim 11 wherein the heat spreadercomprises one or more conducting electrodes that extend from one surfaceto the opposite surface of the heat spreader, wherein the conductingelectrodes are electronically insulated from the heat spreader.
 15. Thesemiconductor package of claim 14 further comprising providing at leastone electrical connection from the capacitance measuring device to theone or more conducting electrodes.
 16. The semiconductor package ofclaim 15 further comprising providing at least one electrical connectionfrom the capacitance measuring device to a region of the heat spreaderthat is electrically insulated from the one or more conductingelectrodes.
 17. The semiconductor package of claim 13 wherein theplurality of electrical leads that electrically connect the conductiveplate to the semiconductor chip include carrier pads.